1. Field of the Invention
The present invention relates to a halftone image recording system for an electrophotographic recording apparatus which records halftone images by black and white dots.
2. Discussion of Background
In a prior art halftone image recording system of the type employing black and white dots, a two-dimensional dot matrix is used as a unit and the number of black dots in the dot matrix is changed to render various densities. The problem encountered with this type of system is that a halftone image with multiple gradations cannot be recorded without increasing the size of the dot matrix and, therefore, without lowering the resolution or increasing the recording time.
The problem stated above may be solved by a system known by the name of Mead system and disclosed in Japanese Patent Laid-Open Publication No. 57-91074. What characterizes the Mead system is the utilization of the effect of overlaying, in detail, reproducing multiple gradations by combining a gradation provided by a number of dots and that provided by overlapping portions of dots. The Mead system, however, is unsuitable for rendering halftone in a low density range and, moreover, it cannot be applied to a raster scan type electrophotographic recording apparatus in which a positive latent image is produced by erasing electrostatic charge in non-image areas of the surface of a photoconductive element.